The existence of small ice particles has remained a highly debatable issue in the cloud physics community. The first measurements of ice cloud particle sizes were obtained in the 1930's with the help of airborne impactors, yet almost eighty years later, researchers have yet to establish a consensus on whether observations of small ice particles with diameters <100 μm represent naturally occurring ice particles, or are the result of shattering of larger ice particles with the measuring probe's arms.
The presence of small ice particles may play a crucial role in the conversion of water vapor into precipitation. Furthermore, this may significantly affect radiation transfer in clouds and eventually affect the radiation budget of the Earth. Currently, small ice particles are included in many weather prediction and climate models, despite the fact that their natural occurrence has not yet been fully demonstrated.
The majority of airborne probes that are designed to measure cloud particles sizes use a laser-based measurement method, e.g. as illustrated in FIGS. 1A and B. The laser is shone between two supporting arms 1 which point into the air stream. As cloud particles with diameters from sub-micron to several centimeters cross through the laser beam 2, the laser light is blocked or scattered (depending on the instrument) by the cloud particles. Changes to the laser beam can be measured by various types of detectors from which the shapes, sizes and concentrations of the cloud particles can be determined. Existing technology uses semi-spherical or rounded arm tips 3 for the particle measurement probes. As shown for example in FIG. 1A, the semi-spherical tips generate large amounts of shattered, splashed and/or bounced particle fragments that are deflected into the sample volume of the probes.
It has been assumed for many years that cloud particles that shattered, splashed or bounced off of the protruding measurement arms, and which subsequently passed through the laser beam, would have an insignificant effect on the measurements of the cloud particle sizes and concentrations. This assumption is no longer deemed to be correct, and significant effort has been made to understand this phenomenon. Data processing methods to correct for the distortions in the natural cloud particle spectra caused by the particle shattering have been developed (Korolev et al., Journal of Atmospheric and Oceanic Technology, 2005, 22:528-542; Lawson et al., Journal of Applied Meteorology and Climatology, 2006, 45:1291-1303) and alternate methods of data collection using non-airborne instrumentation have also been pursued (Mertes et al., Aerosol Science and Technology, 2007, 41:848-864).
Despite these efforts, there continues to be a need for improved ice cloud particle measuring instrumentation that reduces the cloud particle shattering effect previously seen in cloud particle spectra.